I would like to be able to use a trailer, either open-air or enclosed to transport multiple bicycles at the same time. When I google hitch racks the maximum is 5...I am looking for a towable option...do these exist? Where can I find one? Could I make one?

Did you try asking at a car shop? They may know best what auto-trailers exist. Personally, I think you may need a (small) bus to carry that much bikes.
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UoooApr 18 '13 at 6:25

1

If you google Image search "uplift trailer" you'll see loads! I reckon there will be someone offering hireable trailers. You might be able to purchase one second-hand from an uplift service that deals with Downhill Mountain Bikers.
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Mere DevelopmentApr 18 '13 at 9:38

I once constructed a trailer that could in theory handle 8 bikes. Used it for family biking and for Boy Scout activities. I used a standard 4x8 utility trailer as the base, and added removable uprights that the bikes could be strapped to.

The hard part is finding some sort of mounting scheme that can accommodate bikes of different sizes and styles (and do so reasonably cheaply).